The present invention relates to an improved Transmitting Optical Subassembly (TOSA) used for coupling the optical output signal of an optoelectronic transmitter to an optical fiber transmission medium. More particularly, the present invention provides an improved TOSA including output normalization for adjusting the output characteristics of an optical transmitter to fit within predefined optimal parameters.
In the field of data communications, optoelectronic transceivers act as the interface between electrical and optical transmission media. Optical transmitters convert electrical data signals into optical signals which may be transferred over fiber optic cables. Conversely, optical receivers receive optical signals and convert them into electrical signals. Optical transmitters and receivers may be combined into a single device to form an optoelectronic transceiver. A key element of any optical transmitter or receiver is the optical subassembly. In the case of an optical transmitter, the optical subassembly comprises a transmitting optical subassembly, or TOSA, and in the case of an optical receiver, the optical subassembly comprises a receiving optical subassembly, or ROSA. While the present invention may include possible applications involving ROSA, the primary focus of the invention has been to provide a TOSA which acts to normalize the output characteristics of an optical transmitter, and henceforth the discussion will be limited to an improved TOSA.
A TOSA acts as the interface between an electronic data communication medium and an optical data communication medium. In fact, the TOSA occupies the physical space between the optoelectronic transmitter electrical drive circuitry and an optical fiber. The TOSA provides the physical structure to couple the optical output signal of the transmitter to an optical fiber, and acts to align and focus the optical signal onto the end of the fiber such that the light signal enters the fiber and is transmitted to a remote location where the signal can be received and converted back into an electronic signal.
Because the diameter of an optical fiber is quite small, alignment of the fiber with the optical signal is of critical importance to the operation of an optoelectronic transceiver. To facilitate alignment of the optical fiber, the end of a fiber optic cable will often be provided with a specially adapted fiber optic connector. While such connectors are offered in a variety of forms, a common fiber optic connector includes a cylindrical plastic or ceramic connector ferrule having a large diameter relative to the diameter of an optical fiber. The connector ferrule includes a narrow axial hole through which an optical fiber may be inserted and epoxied to the inside of the ferrule. The dimensional tolerances of the ferrule are extremely low such that the outer circumferential surface of the ferrule provides a fixed reference for locating the axial fiber sheathed therein. A TOSA includes cooperating structures for receiving a connector ferrule and aligning the optical fiber with an optical signal emitted by an active optical element housed within the TOSA.
A TOSA generally comprises a cylindrical housing formed of molded plastic or machined from a metal such as stainless steel. A first hollow end of the housing may have a larger diameter than the second end, and is configured to receive a focusing element and an optical package. The optical package itself generally comprises an active optical element such as an LED or a semiconductor laser mounted on a header or substrate. A protective cover including a transparent window to allow the optical signal emitted by the active optical element to radiate from the package may be provided and hermetically sealed to the header. Alternately, the active element may simply be covered by a fast curing resin. Electrical leads extend through the header such that the optical element may be electrically connected to the transmitter's drive circuitry such that data signals may be transferred to the optical element and converted to light signals.
The second end of the TOSA generally comprises a long narrow bore configured to receive a connector ferrule. At the distal end of the bore a narrow passage joins the bore to the first hollow end of the housing, allowing optical signals to pass therebetween. The bore at the second end is precision molded or machined such that the inner diameter of the bore is precisely located relative to the passage. This ensures that when a connector ferrule is inserted into the bore, the optical fiber axially positioned within the ferrule will be located directly over the center of the passage.
To complete the assembly of the TOSA, the optical package is aligned with the focusing element such that the optical signal emitted by the active optical element is focused to a narrow point of light corresponding to the position of the polished end of the optical fiber carried by the connector ferrule inserted into the ferrule receiving bore. Once proper alignment of the optical package is achieved, the optical package may be permanently attached to the TOSA housing. The TOSA may then be connected to the main body of an optoelectronic transmitter. Additional connector apparatus may be provided with the transmitter to releasably retain fiber optic connector ferrules within the TOSA.
LEDs and semi-conductor lasers are often employed as the active optical element in optoelectronic transceivers. The present invention may be employed with optical packages employing any active optical element. However semiconductor lasers, particularly vertical cavity surface emitting lasers (VCSEL) have a number of advantages over LEDs, most having to do with improved speed, efficiency, longevity and reliability. However, employing semiconductor lasers gives rise to other problems which must be addressed. One of the primary difficulties with semiconductor lasers is that each individual laser will have its own unique set of output characteristics. For example, FIG. 1 shows typical output power versus input current curves, or P-I curves, for two individual semiconductor lasers A and B. The X-axis represents the drive current input to the semiconductor lasers, and the Y-axis represents the corresponding optical output power delivered by the lasers. It should be noted that each curve includes a linear region 10a, 10b. The linear region represents the linear operating range of the laser and the slope of the linear portion represents the slope efficiency .eta. of the laser. Thus, .eta.=.DELTA.P/.DELTA.I. By comparing the curves in FIG. 1 it is clear that the slope efficiencies of lasers A, and B are not the same. From the slope of the various linear segments 10a, 10b, it can be seen that .eta..sub.A is greater than .eta..sub.B.
In addition to the variability in the output characteristics of individual semiconductor lasers, the output signal of a TOSA is further dependent on how well the output signal of the active optical element is coupled to the transmission medium, namely in optical fiber. FIG. 1 further includes curve C which represents the output signal of a TOSA employing the semiconductor laser having the output characteristics of curve A. By comparing curves A and C, it can be seen that due to less than 100% optical coupling the output power of the TOSA is significantly reduced for all levels of input current I. Thus, component alignment and effective output coupling have a significant impact on the TOSA output characteristics in addition to variability in the output characteristics of the active optical elements themselves.
The differences between the three curves shown in FIG. 1, including their relative heights and slope efficiencies, have a significant impact on the optical power emitted by a TOSA. For example, in FIG. 1 a uniform DC input current I.sub.Q is supplied. However, the resultant output power P.sub.A, P.sub.B, and P.sub.C varies widely. Furthermore, because of the differing slope efficiencies of the various curves, equal changes in the input current will result in unequal changes in the output power. Thus, for .DELTA.I shown in FIG. 1, the resulting change in the output power .DELTA.P of the optical signals is markedly different for each curve. In other words, as shown in FIG. 1, for a uniform .DELTA.I applied to curves A, B, and C .DELTA.P.sub.A.noteq..DELTA.P.sub.B .noteq..DELTA.P.sub.C.
These variations in the output characteristics raise a significant barrier to designing a standard, reliable optoelectronic transceiver suitable for mass production. Ideally, each optoelectronic transceiver of a particular design will have similar output characteristics. In normal operation, the TOSA will be supplied with a DC quiescent operating current I.sub.Q modulated by a binary data signal having an amplitude .+-..DELTA.I. The output signal will comprise an optical signal having average power P.sub.Q modulated by the data signal i the amount .+-..DELTA.P. The ratio between P.sub.Q +.DELTA.P and P.sub.Q -.DELTA.P is known as the extinction ratio of the modulated optical output signal. It is desirable that every optical transmitter of a particular design have similar, if not identical output characteristics including the average output power P.sub.Q and extinction ratio. By normalizing the output characteristics of a line of optoelectronic transmitters the task of designing a reliable, interchangeable optical receiver is simplified. With optical signals having approximately the same average power and extinction ratio, it is easier for a receiver to distinguish between received binary 1's and binary 0's.
Unfortunately, each TOSA will have its own unique output characteristics. As FIG. 1 graphically demonstrates, applying a uniform input current signal to a number of different TOSAs employing unique semiconductor lasers will result in non-uniform output signals having dissimilar average power and extinction ratios. Past optoelectronic transmitters have attempted to normalize the output characteristics of individual transmitters by adjusting the input current supplied to the semiconductor laser. For example, this technique is disclosed in co-pending Patent Application Ser. No. 08/904,130 assigned to Methode Electronics, Inc. Using this approach the output characteristics of individual lasers are measured and biasing resistors within the transmitter's drive circuitry are altered to change both the quiescent current I.sub.Q and the magnitude of the modulation current .+-..DELTA.I supplied to the lasers. By manipulating these parameters the output signal can be tailored, or normalized, to predetermined optimal levels. In other words, by adjusting the internal biasing resistors, multiple transmitters may be constructed, each having a near identical output signal including the same normalized average output power and extinction ratio.
While the method of normalizing the output characteristics of optoelectronic transceivers by altering the input current supplied to the laser is effective in achieving the desired output results, the process is time consuming and labor intensive. Furthermore, the normalization process described above does not compensate for the bandwidth versus forward current dependency exhibited by some semiconductor lasers. In some cases, it is necessary to boost the forward current supplied to a semiconductor laser to reduce the rise/fall time of the optical signal representing the transitions between binary 1's and binary 0's in order to increase the bandwidth of the transmitter. In such cases the quiescent current and quiescent output power will necessarily be higher than the desired values, and it is not possible to normalize the output signal by adjusting the input current without sacrificing bandwidth. It is desirable that like TOSAs all have the same output signal characteristics regardless of the quality of the alignment of the optical elements therein. Thus, it may be necessary to attenuate the output of a TOSA having strong output coupling to match the output of a TOSA having poor coupling of the laser output signal. Determining the necessary amount of such attenuation must necessarily take place after the optical package has been aligned with the TOSA housing and permanently attached thereto.
There is a need therefore, for a simpler and less expensive optoelectronic transmitter, having normalized output characteristics substantially matching a preset standard. Output normalization should be provided without requiring alteration to the transmitter circuitry, and should be possible at any time after an optical package has been installed and aligned with the TOSA.